Real-Time and Atomic-Level Studies of the Growth, Phase Transformations and Stability of Two-Dimensional Pnictogens

Les pnictogènes, ou éléments du groupe VA sont composés de l’azote (N), du phosphore (P), de l’arsenic (As), de l’antimoine (Sb) et du bismuth (Bi). Grâce à leur configuration électronique de valence (ns2 np3), ils adoptent une hybridation sp3 et forment trois liens covalents en phase solide. Dû à cette configuration électronique unique, les pnictogènes, avec le graphite, sont les seuls matériaux élémentaires à se cristalliser en structures en couches van der Waals (vdW) et quasi-vdW. Typiquement, les éléments légers du groupe VA forment des solides de la phase orthorhombique A17 tandis que les éléments lourds préfèrent la phase rhomboédrique A7. Avec leur structure en couches, les pnictogènes sont des candidats idéaux pour former des matériaux bidimensionnels (2D). En effet, des couches 2D de phosphore noir (A17) ont été obtenues par exfoliation de cristaux massifs en 2014. Le 2D-P a été identifié comme étant un semiconducteur à mobilité élevée ayant des propriétés de transport anisotropes et possédant une bande interdite directe dont l’amplitude augmente graduellement de 0.3 à 2 eV en passant du matériau massif à des couches d’épaisseur atomique. Toutefois, au début de ce projet, la plupart des pnictogènes 2D n’existaient que sous forme de prédictions théoriques. En effet, on prédisait que les matériaux 2D légers du groupe VA seraient des semiconducteurs à large bande interdite, tandis que les pnictogènes lourds subiraient plusieurs transitions de phase électronique et topologique lorsqu’ils approcheraient des épaisseurs atomiques. Cette thèse vise à développer la synthèse de nouveaux pnictogènes 2D et à élucider les mécanismes gouvernant leur croissance, leur stabilité thermodynamique, ainsi que leurs propriétés physiques de base. La croissance par épitaxie par jets moléculaires (MBE) sur des substrats semiconducteurs et vdW, les transitions de phase et la décomposition thermique de matériaux 2D du groupe VA a été étudiée en temps réels par microscopie électronique à faible énergie (LEEM). De plus, leurs propriétés structurelles, électroniques et thermodynamiques ont été élucidées par la combinaison de calculs ab initio avec des mesures de diffraction d’électrons lents (LEED), de microscopie par effet tunnel (STM), de microscopie électronique en transmission à balayage (STEM) et de microscopie de photoélectrons par rayons-X (XPEEM). ----------Abstract Pnictogens, also known as group VA elements, are comprised of nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi). With their ns2 np3 valence electronic configuration, pnictogens tend to adopt a sp3 hybridization and form three covalent bonds in elemental solids. This unique electronic configuration makes them the only elemental materials, alongside graphite, to crystallize in van der Waals (vdW) and quasi-vdW layered structures. Light group VA elements tend to assemble in the orthorhombic phase (A17) and heavier group VA elements prefer the rhombohedral phase (A7). With their layered structures, pnictogens are ideal candidates to form two-dimensional (2D) materials. In fact, 2D black phosphorus (A17) has been exfoliated from bulk crystals in 2014 and was identified as a high mobility 2D semiconductor with a thickness-dependent direct band gap varying between 0.3-2 eV and displaying interesting anisotropic transport properties. However, by the time this project was initiated, most 2D pnictogens existed only in the realm of theoretical predictions. In fact, it was hypothesized that light group VA 2D materials would be large band gap semiconductors, whereas heavy 2D pnictogens were predicted to exhibit several electronic and topological transitions at near atomic thicknesses. This thesis aims at developing growth methods for novel 2D pnictogens allotropes and at establishing an atomic-level understanding of the mechanisms governing their growth, thermodynamic stability and basic physical properties. The molecular beam epitaxy (MBE) growth on semiconducting and vdW substrates, the phase transformations and the thermal decomposition of group VA 2D materials was studied in real-time using low-energy electron microscopy (LEEM). Furthermore, their structural, electronic and thermodynamic properties were elucidated by a combination of ab initio calculations, low-energy electron diffraction (LEED), scanning tunnelling microscopy (STM), scanning transmission electron microscopy (STEM) and synchrotron-based X-ray photoemission microscopy (XPEEM)

Direct Spectroscopic Observation of Cross-Plane Heat Transfer in a Two-Dimensional Van der Waals Heterostructure

Two-dimensional (2D) transition metal chalcogenides (TMDs) have drawn significant attention in recent years due to their extraordinary optical and electronic properties. As heat transfer plays an important role in device performance, various methods such as optothermal Raman spectroscopy and time-domain thermoreflectance have been developed to measure the thermal conductivity and interfacial thermal conductance in 2D van der Waals (vdW) heterostructures. Here, we employ the vibrational-pump-visible-probe (VPVP) spectroscopy to directly visualize the heat transfer process in a heterostructure of multilayer h-BN and monolayer WS2. Following an impulsive vibrational excitation of h-BN in the mid-infrared, we probe the heat transfer from h-BN through WS2 and finally to the substrate from the subpicosecond to the submicrosecond timescale. The interfacial thermal conductance of the h-BN/WS2 and WS2/SiO2 interfaces is obtained by corroborating the experiments with heat transfer calculations based on the Fourier’s law of heat conduction. Our study demonstrates an alternative, time-resolved optical method to measure cross-plane heat dissipation and opens up a new pathway to investigate the interlayer electron–phonon and phonon–phonon interactions in vdW heterostructures

Pnictogens Allotropy and Phase Transformation during van der Waals Growth

Pnictogens have multiple allotropic forms resulting from their ns2 np3 valence electronic configuration, making them the only elemental materials to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light group VA elements are found in the layered orthorhombic A17 phase such as black phosphorus, and can transition to the layered rhombohedral A7 phase at high pressure. On the other hand, bulk heavier elements are only stable in the A7 phase. Herein, we demonstrate that these two phases not only co-exist during the vdW growth of antimony on weakly interacting surfaces, but also undertake a spontaneous transformation from the A17 phase to the thermodynamically stable A7 phase. This metastability of the A17 phase is revealed by real-time studies unraveling its thickness-driven transition to the A7 phase and the concomitant evolution of its electronic properties. At a critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle transition from AB to AA stacked alpha-antimonene followed by a gradual relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of this intermediate phase is found to be determined by surface self-passivation and the associated competition between A7- and A17-like bonding in the bulk. These results highlight the critical role of the atomic structure and interfacial interactions in shaping the stability and electronic characteristics of vdW layered materials, thus enabling a new degree of freedom to engineer their properties using scalable processes